1. Field of the Invention
An aspect of the present invention relates to a semiconductor laser apparatus and a method for mounting a semiconductor laser apparatus, particularly to an array-type semiconductor laser apparatus, more particularly to a semiconductor laser light source to be used in a laser beam printer or a laser-type copying machine, and especially to an array-type semiconductor laser, in which a plurality of light emitting points are arrayed at a narrow spacing of 50 μm or less between laser light sources.
2. Description of the Related Art
In a laser beam printer, the printing speed can be increased in proportion to the number of the simultaneous beams. For example, an array-type semiconductor laser having a structure, as shown in FIG. 11, may be used.
In FIG. 11: numeral 1 designates an n-type GaAs substrate on which a double hetero-structure constituting a semiconductor laser is grown through the epitaxial crystal growth; numeral 2 an n-type cladding layer formed over the substrate 1 and made of AlGaInP; numeral 3 an active layer made of AlGaInP; numeral 4 a cladding layer made of p-type AlGaInP; numeral 5 a current blocking (block) layer made of n-type GaAs; and numeral 6 a contact layer. The mixed crystal composition ratio is set so that the energy gap of the AlGaInP active layer 3 may have an energy gap smaller than those of the AlGaInP cladding layers 2 and 4, thereby constituting a double hetero-structure. The semiconductor laser is required to make single-mode oscillations for the application, in which a laser beam for a printer is focused by using an optical system. In order to attain single-mode oscillations and to reduce an electric current necessary for the laser oscillations, a p-type AlGaInP layer 4 is selectively etched to form a ridge while leaving its stripe portion, and the current blocking (block) layer 5 is selectively grown by using of an amorphous film such as SiNx. As a result, only the portion having the ridge becomes a conductive region thereby forming a stripe waveguide 113. If this structure is cleaved to form a reflecting mirror, it forms an optical resonator between itself and the stripe waveguide which makes an optical gain when energized, thereby forming a facet light-emitting laser structure. After the SiNx film is removed, the GaAs contact 6 is crystally grown to form a laser chip having a p-electrode 7 and an n-electrode 8. In the case of an array laser, as shown in FIG. 11, a plurality of stripe waveguides 113 are formed in the single chip, and the p-electrode 7 is separately provided to correspond to each of the stripe waveguides 113.
The laser chip is fixed on a submount 9 made of AlN or SiC having a large thermal conductivity coefficient by soldering, as indicated by 10, the electrodes of the laser chip. In the semiconductor laser element having only the single light-emitting region, the laser chip is fixed to the submount so that the face (corresponding to the face of the upper side of FIG. 11) on which the epitaxial growth is performed of the laser chip is adhered to the submount surface. This assembly mode is generally called the “downward junction-face assembly, because the face having the p-n junction of the laser chip mounted over the submount is positioned on the lower side of the chip. With this constitution, the heat, which is generated in the p-n junction in the epitaxial grown film, is efficiently dissipated to the submount made of an insulator of a high thermal conductivity.
In the array laser, too, the downward junction-face assembly is desired, if only the heat dissipation is considered. In the array-type laser required to have the electric insulation between the elements, however, the downward junction-face assembly is liable to invite the short-circuiting between the elements, thereby causing a problem that the mass productivity is poor. Therefore, this array laser is generally mounted in the upward junction-face assembly, in which the back-face electrode (corresponding to the n-electrode 8 of FIG. 11) of the laser chip is soldered to the submount. In this case, the energization of the surface electrode (corresponding to the p-electrode 7 of FIG. 11) is realized by boding a gold wire 11 to each of the divided electrodes (for example, I. “Yoshida et al., Jpn. J. Appl. Phys. Vo. 34 (1995) pp. 4803” and “R. Geel et al., Electron. Lett. Vo. 28 (1992) pp. 1420”).
For necessities of the high image quality, the high speed and the small size of a laser printer, however, there is further desired a laser element having more-stable optical output at a high temperature. For satisfying the desire, the necessity for adopting the downward junction-face assembly is rising. In case the spacing of the array lasers is wide, the problem of a short-circuiting is hard to arise, so that the laser chip can be assembled with the junction face being downward, as shown in FIG. 12.
In case the spacing of the array lasers is narrow (as specified by about 50 μm or less, for example), the production yield drops due to the occurrence of the short-circuiting. Due to the thermal expansion coefficient difference between the submount and the semiconductor chip, moreover, a stress occurs in the element in the cooling procedure from the solder solidification to the room temperature, thereby causing problems in the reliability of the semiconductor laser or the instability of the oscillating wavelength.
JP-2004-14659-A discloses a method for preventing the influences of the stress in the element having the single light emitting region. An exemplary structure is shown in FIG. 13.
In FIG. 13, 601 designates a semiconductor laser element; 601a designates a ridge structure portion; 601b designates a recessed portion; 606 designates a solder layer; 608 designates a mounting substrate; 611 and 612 designate element-side electrode films; 611a designates an electrode film of the ridge structure portion 601a; 611b designates an electrode film of the recessed portion 601b; 611c designates an electrode film of a horizontal portion; 613 designates a board-side electrode film; 618 designates an active layer; 622 designates an element-side solder wet suppressing film; and 623 designates a board-side solder wet suppressing film.
According to a structure shown in FIG. 13, the vicinity of the stripe waveguide is not fixed with a solder but only the region spaced properly from the stripe waveguide (that is, the portion spaced at a proper distance from a region of the active layer of the laser) is fixed by the solder to the submount.
According to JP-2004-14659-A, the stress at the time of mounting the semiconductor laser into the submount exerts adverse influences upon the laser light emission of the laser element, if the laser element is fixed in the region within about 20 μm from the junction portion of the solder, as shown in FIG. 14, because the stress of the active layer increases in that region. Therefore, the junction portion is soldered at a position spaced by more than 20 μm. Thus, it is disclosed that the influences of the junction stress on the stripe waveguide 113 can be reduced, and that the heat can also be dissipated from the laser element by making use of the thermal conduction by the electrode.
As described hereinbefore, the problem that the stress occurs in the downward junction face assembly in the cooling procedure from the solder solidifying temperature to the room temperature due to the thermal expansion coefficient difference between the submount and the semiconductor substrate, thereby degrading the reliability of the semiconductor laser and the stability of the oscillation wavelength can be solved by setting the distance from the stripe waveguide (as designated by numeral 113 in FIG. 1) to the region (as designated by numeral 129 in FIG. 1) fixed by soldering on the submount, to 20 μm or more (or far more, if possible). According to JP-2004-14659-A, this case raises no problem in the heat dissipation, because of the presence of a thermal conduction by the surface electrode.
In the array-type semiconductor laser for the laser printer, however, the temperature of the stripe waveguide 113 is raised by the Joule's heat accompanying the energization so that the phenomenon, as called the “droop”, for the optical output to decrease for a while raises a problem. In this phenomenon, the temperature of the stripe waveguide 113 rises to increase the threshold current of the semiconductor laser, so that the laser output drops even in the state of a drive in a constant current. The stable optical output is required from a low output to a high output (from 1 mW to 20 mW, for example) thereby raising a serious problem in a printer array laser, in which a plurality of exothermic regions are present in a common chip. This problem makes it necessary to suppress the distance from the aforementioned stripe waveguide 113 to the region 129 fixed on the submount by the solder, to at least 10 μm or less. Thus, it is difficult to solve the problem by the proposal in JP-2004-14659-A.